Tea is something of a bonus character, and only appears in bonus pages. She is a non-canon student at the Court whose . skill in fourth-wall breaking has also been used to explain parts of the Gunnerverse or remind readers of past events.
Tom has stated that she is not a representation of himself (something that some readers argued about since Tom's DeviantArt account is Tea-san), and the two have even been seen side-by-side in all the "Summer Holidays" pages (though Tom's face is never shown).
Tea existed pre-Gunnerkrigg Court and you can find her in Tom's DeviantArt page.